Heat-sensitive recording materials which utilize an electron donating colorless dye and an electron accepting compound are well known, and are disclosed, e.g., in British Pat. No. 2,140,449.
Properties which heat-sensitive recording materials must be possessed of at the minimum are (1) to have satisfactorily high color density of the developed image, (2) to undergo no color development upon storage prior to use, (3) to yield a colored product which retains sufficient fastness after color development, and so on. However, recording materials which simultaneously satisfy all of these requisites to perfection have not yet been obtained.
In particular, studies on the property (1) described above have been made energetically with the speeding-up of heat-sensitive recording systems in recent years.
One of measures which have been attempted with the intention of conferring the property (1) on heat-sensitive recording materials involves the use of an electron accepting compound having a melting point lower than 120.degree. C. In another measure, a combination of an organic acid and a phenolic compound is used as an electron accepting substance. In addition, the use of a polyvalent metal salt of a compound having an alcoholic hydroxyl group, the use of a copolymer prepared from hydroxyethyl cellulose and maleic anhydride, the addition of waxes, the addition of thioacetoanilide, phthalonitrile, acetamide, di-.beta.-naphthyl-p-phenylenediamine, a fatty acid amide, acetoacetic acid anilide, diphenylamine, benzamide, a nitrogen-containing organic compound like carbazole, a thermofusible substance such as 2,3-di-m-tolybutane, 4,4'-dimethylbiphenyl, etc., or a carboxylic acid ester such as dimethyl isophthalate, diphenyl phthalate, dimethyl terephthalate, etc. as a sensitizer, and the addition of hindered phenols have been examined for efficiency in heightening the color density of developed image.
However, heat-sensitive recording materials produced utilizing such measures are insufficient in color density of the developed image and coloring speed.
On the other hand, developments of various aromatic ethers are proceeded, and it has been found that phenyl phenoxyacetate, diphenoxyethane, and the like are particularly excellent in sensitizing effect (e.g., Japanese patent application (OPI) Nos. 58,789/86, 123,581/86, and 176,544/86, etc.). (The term "OPI" as used herein refers to a "published unexamined Japanese patent application.") However, it has also been found that these ethers are attended by some problems.
For instance, sensitizers having an ester moiety derived from phenol, such as phenyl phenoxyacetate, suffer from a difficult point in their own keeping stability with elapse of time.
Also, diether compounds having good symmetry, such as diphenoxyethane, have a defect that when coated on a sheet of paper it is difficult to obtain sufficient keeping stability with elapse of time.